1. Field of the Invention
This invention relates generally to pilot operated pressure relief valves to permit fluid flow from a pressure vessel at a predetermined fluid pressure in the pressure vessel, and is particularly directed to such relief valves adapted for a low fluid pressure operation. The main safety relief valve member has a diaphragm actuator connected thereto for movement with the main valve member and a pilot control valve is effective to control fluid pressure for the dome chamber on one side of the diaphragm and to control fluid pressure for the underdome chamber on the opposite side of the diaphragm.
2. Description of the Prior Art
Heretofore, in pilot operated pressure relief valves, the main valve member of the relief valve has utilized a fluid pressure responsive actuator including a diaphragm having a dome chamber adjacent the outer or upper side of the diaphragm and an underdome chamber adjacent the opposed lower or inner side of the diaphragm. Upon actuation of the pilot control valve, line or inlet fluid pressure is communicated to the underdome from the pressure vessel to provide a boost or auxiliary force for opening of the main valve member to overcome the seating force resulting from such factors as (1) the weight of the moving members, and (2) the resistance force of the bending of the diaphragm. Such an arrangement permits a minimum set pressure to be obtained as low as about 1/2 to 1 inch water column (wc). Thus, the use of inlet or lading fluid for the underdome of the main diaphragm effectively moves the diaphragm and connected main valve member at very low fluid pressures.
U.S. Pat. No. 5,555,910 dated Sep. 17, 1996, shows a pilot operated safety relief valve adapted for low pressure operation and providing line or inlet pressure to an underdome chamber beneath the diaphragm upon actuation of the pilot valve at a low fluid pressure. However, an underdome control valve for the underdome chamber separate from the pilot valve for the main valve member is required. The pilot valve is in communication with the dome chamber for controlling the movement of the main valve member. The separate underdome control valve is in fluid communication with the underdome chamber and has an underdome valve member to control fluid communication of the underdome chamber with the inlet or line fluid pressure. The separate underdome control valve member normally effects venting of the underdome chamber to atmosphere during normal operation. However, upon reacting of the set pressure, the separate underdome valve member permits communication of inlet fluid pressure to the underdome chamber to boost opening of the main valve member.
It is desirable that a single control valve be provided to control fluid flow to both the dome chamber and opposed underdome chamber of a diaphragm actuator for the main valve member of a pilot operated safety relief valve.
It is an object of the present invention to provide a pilot operated safety relief valve having a diaphragm actuator in which a single pilot valve is effective to control fluid flow for both the dome chamber and opposed underdome chamber of the diaphragm actuator for the main valve member without any auxiliary control valves.